This invention relates to optical systems, and, more particularly, to an infrared optics system using inexpensively produced mirrors of intermediate optical quality.
Passive optical components are widely used in optical systems to direct the path of light beams. Such optical components can be reflective, such as a mirror, or refractive, such as a lens. Light is reflected from the surface of a reflective component, but must pass through the body of a refractive component. When used for infrared applications, refractive components require the use of infrared-transmissive materials, which are expensive by comparison with the materials that can be used for reflective components. This higher cost of refractive components is a particular disadvantage when the optical system is intended to be inexpensive, with intermediate optical performance. The present invention is concerned with low-cost optics systems that are made using mirrors, and the mirrors themselves.
Mirrors are usually made by fabricating a mirror body of a required shape but which is not sufficiently reflective. The fabrication of the mirror body may be straightforward, but can also be quite difficult where a complex mirror structure is required. A reflective coating of aluminum or other material is applied to a surface of the mirror body. Mirror bodies can also be made of a material which itself is highly reflective, and whose surface is polished. Mirrors are available in various grades according to their degree of optical perfection and other properties.
Usually, the higher the quality of the mirror, the greater its cost to produce. The selection of a mirror for a particular optical system is based upon the required optical properties for that system, as well as mechanical, weight, and other requirements. As with other system elements, it is preferred that the mirror not be overdesigned with a level of capability greater than that required by the system. Such higher capability usually is associated with higher cost and may be associated with higher weight and lower strength.
One type of mirror is made of high-quality glass that has been provided with a reflective surface coating. Glass mirrors require polishing. If the shape of the reflective surface is not flat or spherical, the polishing can be difficult and expensive. Glass mirrors are rather fragile and therefore must be specially supported in shock-resistant mounts for many applications, which adds to the cost and weight of the optical system. It can be difficult to fabricate complex mirror structures from glass.
Mirrors have also been made of plastic whose surface has been coated with a reflective coating. The available plastic mirrors have significantly lower optical quality than glass mirrors. On the other hand, they are more resistant to breakage and cost less to produce in mass production quantities. Plastic mirrors are subject to warpage due to bending upon mounting in an optical system because of their lower structural strength. Even though light is reflected from the surface of the mirror, the mirror itself must have good structural strength in order to avoid warpage during service that may completely degrade its optical performance. Temperature effects such as temperature gradients across the mirror can also degrade the performance of a plastic mirror. Due to their combination of characteristics, the available plastic mirrors simply are not suitable for a number of medium-performance, low-cost potential applications such as in automobiles, security systems, and certain types of sensors such as infrared-detecting sensor systems.
Mirrors can be made of other materials to which a reflective coating may be applied. Metals, such as aluminum or copper, or ceramics can be used in mirrors. Some metals can be used without a surface coating. Metal mirrors can give good optical performance, but they are typically expensive and heavy. Coated ceramic mirrors are expensive to produce and are relatively fragile.
There is a need for an improved mirror having optical performance intermediate that of the available high-performance glass, metal, or ceramic mirrors, and the low-performance plastic mirrors. A mirror of this type would be used in optical systems wherein a moderate degree of optical performance is required. Such a mirror should also be robust, light weight, inexpensive to produce in production quantities, and reasonably stable in service. This need is particularly acute for infrared optical systems, where suitable mirrors have been previously unavailable where such intermediate-level quality is acceptable. The present invention fulfills this need, and further provides related advantages.